I use an Osprey Stratos 40 and can't recommend it enough. I have had it for at least 1.5 years and I use it for everything from travel, hiking, and my 3 - 4 days a week bike commute. The pack has a light internal frame and suspension that makes the usable space kind of weird, but I think you could probably get a laptop (in a separate sleeve), clothes, and lunch into it. The pack itself is not 'waterproof', but you can buy a separate rain cover that works fairly well. I live in TX and ride in 100+F temps in the summer. Yes, my back gets sweaty, but I don't think anything short of strapping an AC on your back will solve that once you get to 100F. The Talon is very good too. It's just a smaller (half the capacity) of what I have.

I just remembered one thing ... If you go with a backpack I recommend you fill it full and try it with your helmet. My gf has a short neck and some of the outdoorsy packs (Osprey) extend above her shoulders and prevent her from fully raising her head (helmet comes in contact with pack) while she's riding.